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Ghost Galaxies

"Small ghost galaxies, devoid of stars but harboring dense clumps of invisible matter, may outnumber the entire population of luminous galaxies in
the universe."

If ghost galaxies contain no stars at all, how do we know they are there? By extrapolation! In recent years, astronomers have been able to detect dwarf galaxies
lit by just a few luminous stars. These faint, miniscule galaxies are kept from flying apart by the gravitational pull of invisible dark matter. In fact, the density
of dark matter in dwarf galaxies is a hundred times that in our bright, normal-size Milky Way. Further, the more dwarfish and dimmer a galaxy, the denser its
dark matter and the more of them there are in the universe. Now for the promised extrapolation.

J. Kormendy and K.C. Freeman take things one step further, concluding that the universe is flooded with sub-dwarf galaxies that are thick with dark matter,
and without enough luminous stars for us to see them in our telescopes. These ghost galaxies are only 1/10,000 as massive as the bright galaxies like the
Milky Way but much more common.